1. MUSLCES PART 2
HOW DO THEY WORK?

2. Microanatomy of a Muscle Fiber

3. Note: I did not like the pictures in your text for this part
Note: I did not like the pictures in your text for this part. So I have found other pictures I like better.
Video on general structure of skeletal muscle

4. transverse (T) tubules sarcoplasmic reticulum
Microanatomy of a Muscle Fiber (Cell)
transverse (T) tubules
sarcoplasmic reticulum
sarcolemma
terminal cisternae
mitochondria
myoglobin
thick myofilament
thin myofilament
myofibril
triad
nuclei

5. Similar to fig 6.3 pg 183 myofibril Muscle fiber sarcomere Z-line
Thin filaments
Thick filaments
Myosin molecule of
thick myofilament
Thin myofilament

6. Sarcomere A band Z line Z line I band Zone of overlap M line
H zone
I band
Zone of overlap
Zone of overlap
M line
Thin myofilaments
Thick myofilaments

7. Myofilaments of sarcomere
Z line
M line
Z line
Thin myofilaments
Thick myofilaments

8. General view of muscle contraction

9. Thin Myofilament
(myosin binding site)

10. Thick myofilament
(has ATP & actin binding site)
*Play IP sliding filament theory p.5-14 for overview of thin & thick filaments

11. Sliding Filament Theory
Myosin heads attach to actin molecules (at binding (active) site)
Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere
Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

12. Sliding filament theory

13. Once a muscle fiber begins to contract, it will contract maximally
Play IP sliding filament theory p. 28
As sarcomeres shorten, myofibril shortens. As myofibrils shorten, so does muscle fiber
Once a muscle fiber begins to contract, it will contract maximally
This is known as the “all or none” principle

14. Physiology of skeletal muscle contraction
Skeletal muscles require stimulation from the nervous system in order to contract
Motor neurons are the cells that cause muscle fibers to contract
cell body
dendrites
Synaptic terminals (synaptic end bulbs)
axon
telodendria
(motor neuron)

15. Simple video of neuromuscular contraction

16. Neuromuscular junction
telodendria
Synaptic terminal (end bulb)
Synaptic vessicles containing Ach
Synaptic cleft
Neuromuscular junction
Motor end plate
of sarcolemma

17. Overview of Events at the neuromuscular junction
An action potential (AP), an electrical impulse, travels down the axon of the motor neuron to the end bulbs (synaptic terminals)
The AP causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (Ach) into the synaptic cleft
Ach diffuses across the synaptic cleft & binds to Ach receptors on the motor end plate
The binding of Ach to its receptors causes a new AP to be generated along the muscle cell membrane
Immediately after it binds to its receptors, Ach will be broken down by Acetylcholinesterase (AchE) – an enzyme present in the synaptic cleft

18. Action potential
Arrival of an action potential at the synaptic terminal
Axon
Arriving action potential
Synaptic terminal
Sarcolemma
Vesicles
ACh
Synaptic
cleft
AChE molecules
ACh
receptor
site
Muscle
fiber
Sarcolemma of
motor end plate
An action potential (AP), an electrical impulse, travels down the axon of the motor neuron to the end bulbs (synaptic terminals)
Figure 7-4(b-c)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

19. Action potential
Arrival of an action potential at the synaptic terminal
Axon
Arriving action potential
Synaptic terminal
Sarcolemma
Vesicles
ACh
Synaptic
cleft
AChE molecules
ACh
receptor
site
Muscle
fiber
Sarcolemma of
motor end plate
Release of acetylcholine
Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft.
The AP causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (Ach) into the synaptic cleft
Figure 7-4(b-c)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

20. Action potential
Arrival of an action potential at the synaptic terminal
Axon
Arriving action potential
Synaptic terminal
Sarcolemma
Vesicles
ACh
Synaptic
cleft
AChE molecules
ACh
receptor
site
Muscle
fiber
Sarcolemma of
motor end plate
ACh binding at the motor and plate
Ach diffuses across the synaptic cleft & binds to Ach receptors on the motor end plate
Release of acetylcholine
Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft.
The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell.
Na+
Na+
Na+
Figure 7-4(b-c)
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

21. The binding of Ach to its receptors causes a new AP to be generated along the muscle cell membrane
Immediately after it binds to its receptors, Ach will be broken down by Acetylcholinesterase (AchE) – an enzyme present in the synaptic cleft

22. Play IP neuromuscular junction p. 14
Action potential
Arrival of an action potential at the synaptic terminal
Axon
Arriving action potential
Synaptic terminal
Sarcolemma
Vesicles
ACh
Synaptic
cleft
AChE molecules
ACh
receptor
site
Muscle
fiber
Sarcolemma of
motor end plate
ACh binding at the motor and plate
Appearance of an action potential in the sarcolemma
Release of acetylcholine
Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft.
The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell.
An action potential spreads across the surface of the sarcolemma. While this occurs, AChE removes the ACh.
Action
potential
Na+
Na+
Na+
Play IP neuromuscular junction p. 14

23. Physiology of Skeletal Muscle Contraction
Once an action potential (AP) is generated at the motor end plate it will spread like an electrical current along the sarcolemma of the muscle fiber
The AP will also spread into the T-tubules, exciting the terminal cisternae of the sarcoplasmic reticula
This will cause Calcium (Ca+2 ) gates in the SR to open, allowing Ca+2 to diffuse into the sarcoplasm
Calcium will bind to troponin (on the thin myofilament), causing it to change its shape. This then pulls tropomyosin away from the active sites of actin molecules.
The exposure of the active sites allow the sliding of the filaments
Table 7-1

24. Physiology of skeletal muscle contraction – events at the myofilaments
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Active site
Ca2+
Tropomyosin
Actin +
ADP
ADP P P + P +
Myosin reactivation
Cross bridge detachment
Pivoting of myosin head
ADP
ATP
ADP + P + P
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
ADP P +
ATP
ADP + P
Figure 7-5
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

25. Resting sarcomere
Active-site exposure
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
Ca2+
Ca2+
Tropomyosin
Actin
Active site
ADP
ADP P + P +
Calcium (Ca+2 ) gates in the SR open, allowing Ca+2 to diffuse into the sarcoplasm
Calcium will bind to troponin (on the thin myofilament), causing it to change its shape.
This then pulls tropomyosin away from the active sites of actin molecules.
Figure 7-5
3 of 7
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

26. Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ +
ADP
ADP P P + P +
Myosin heads are “energized” by the presence of ADP + PO43- at the ATP binding site (energy is released as phosphate bond of ATP breaks)
Once the active sites are exposed, the energized myosin heads hook into actin molecules forming cross-bridges
Figure 7-5
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

27. The ADP & phosphate group are released from the myosin head
Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + P +
Sarcoplasm P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Ca2+
Tropomyosin
Actin
Active site +
ADP
ADP P P + P +
Pivoting of myosin head
Using the stored energy, the attached myosin heads pivot toward the center of the sarcomere
The ADP & phosphate group are released from the myosin head
ADP + P
Ca2+
Ca2+
ADP + P
Figure 7-5
5 of 7
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

28. Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP + +
Sarcoplasm P P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Active site
Ca2+
Tropomyosin
Actin +
ADP
ADP P P + P +
A new molecule of ATP binds to the myosin head, causing the cross bridge to detach from the actin strand
The myosin head will get re-energized as the ATP  ADP+P
Cross bridge detachment
Pivoting of myosin head
ATP
ADP + P
Ca2+
Ca2+
Ca2+
Ca2+
ATP
ADP + P
As long as the active sites are still exposed, the myosin head can bind again to the next active site

29. Resting sarcomere
Active-site exposure
Cross-bridge formation
ADP
Myosin head
ADP +
Sarcoplasm P + P
Troponin
ADP + P
Ca2+
Ca2+
Ca2+
ADP
Tropomyosin
Actin
Active site
Ca2+ +
ADP
ADP P P + P +
Myosin reactivation
Cross bridge detachment
Pivoting of myosin head
ADP
ATP
ADP + P + P
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
ADP P +
ATP
ADP + P
IP Sliding filament theory – p single cross bridge; p. 27 multiple cross bridges
PLAY
Figure 7-5
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

30. Physiology of Skeletal Muscle Contraction
If there are no longer APs generated on the motor neuron, no more Ach will be released
AchE will remove Ach from the motor end plate, and AP transmission on the muscle fiber will end
Ca+2 gates in the SR will close & Ca+2 will be actively transported back into the SR
With Ca+2 removed from the sarcoplasm (& from troponin), tropomyosin will re-cover the active sites of actin
No more cross-bridge interactions can form
Thin myofilaments slide back to their resting state
Table 7-1

31. Key Note
Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs. The trigger for contraction is the calcium ions released by the SR when the muscle fiber is stimulated by its motor neuron. Contraction is an active process; relaxation and the return to resting length is entirely passive.

32. These physiological processes describe what happen at the cellular level – how skeletal muscle fibers contract
But what about at the organ level? How do skeletal muscles (like your biceps brachii) contract to create useful movement?

33. Skeletal muscles are made up of thousands of muscle fibers
A single motor neuron may directly control a few fibers within a muscle, or hundreds to thousands of muscle fibers
All of the muscle fibers controlled by a single motor neuron constitute a motor unit

34. small motor units  precise control (e.g. eye muscles
The size of the motor unit determines how fine the control of movement can be –
small motor units  precise control (e.g. eye muscles
large motor units gross control (e.g. leg muscles)
Recruitment is the ability to activate more motor units as more force (tension) needs to be generated
There are always some motor units active, even when at rest. This creates a resting tension known as muscle tone, which helps stabilize bones & joints, & prevents atrophy
PLAY
Play IP Contraction of motor units p. 3-7

35. When skeletal muscles contract, they may produce two types of contractions:
Isotonic contraction
Isometric contraction
Iso means= same or equal

36. Isotonic contraction – as tension increases (more motor units recruited), length of muscle changes usually resulting in movement of a joint. The tension (load) on a muscle stays constant (iso = same, tonic = tension) during a movement. (Example: lifting a baby, picking up object, walking, etc. )

37. Isometric contraction – no change in length of muscle even as tension increases. The length of a muscle stays constant (iso = same, metric = length) during a “contraction” (Example: holding a baby at arms length, pushing against a closed door.)
Necessary in everyday life to counteract effects of gravity (e.g. postural muscles keeping head up)